The thermally activated delayed fluorescence (TADF) mechanism has recently attracted significant interest in the field of organic light-emitting diodes (OLEDs). TADF relies on the presence of a very small energy gap between the lowest singlet and triplet excited states. Here, we demonstrate that time-dependent density functional theory in the Tamm-Dancoff approximation can be very successful in calculations of the lowest singlet and triplet excitation energies and the corresponding singlet-triplet gap when using nonempirically tuned range-separated functionals. Such functionals provide very good estimates in a series of 17 molecules used in TADF-based OLED devices with mean absolute deviations of 0.15 eV for the vertical singlet excitation energies and 0.09 eV [0.07 eV] for the adiabatic [vertical] singlet-triplet energy gaps as well as low relative errors and high correlation coefficients compared to the corresponding experimental values. They significantly outperform conventional functionals, a feature which is rationalized on the basis of the amount of exact-exchange included and the delocalization error. The present work provides a reliable theoretical tool for the prediction and development of novel TADF-based materials with low singlet-triplet energetic splittings.

A new design for second near-infrared window (NIR-II) molecular fluorophores based on a shielding unit-donor-acceptor-donor-shielding unit (S-D-A-D-S) structure is reported. With 3,4-ethylenedioxy thiophene as the donor and fluorene as the shielding unit, the best performance fluorophores IR-FE and IR-FEP exhibit an emission quantum yield of 31% in toluene and 2.0% in water, respectively, representing the brightest organic dyes in NIR-II region reported so far.

Organic fluorophores have been widely used for biological imaging in the visible and the first near-infrared windows. However, their application in the second near-infrared window (NIR-II, 1000-1700 nm) is still limited mainly due to low fluorescence quantum yields (QYs). Here, we explore molecular engineering on the donor unit to develop high performance NIR-II fluorophores. The fluorophores are constructed by a shielding unit-donor(s)-acceptor-donor(s)-shielding unit structure. Thiophene is introduced as the second donor connected to the shielding unit, which can increase the conjugation length and red-shift the fluorescence emission. Alkyl thiophene is employed as the first donor connected to the acceptor unit. The bulky and hydrophobic alkyl thiophene donor affords larger distortion of the conjugated backbone and fewer interactions with water molecules compared to other donor units studied before. The molecular fluorophore IR-FTAP with octyl thiophene as the first donor and thiophene as the second donor exhibits fluorescence emission peaked at 1048 nm with a QY of 5.3% in aqueous solutions, one of the highest for molecular NIR-II fluorophore reported so far. Superior temporal and spatial resolutions have been demonstrated with IR-FTAP fluorophore for NIR-II imaging of the blood vessels of a mouse hindlimb.

The significantly reduced tissue autofluorescence and scattering in the NIR-II region (1000-1700 nm) opens many exciting avenues for detailed investigation of biological processes in vivo. However, the existing NIR-II fluorescent agents, including many molecular dyes and inorganic nanomaterials, are primarily focused on complicated synthesis routes and unknown immunogenic responses with limited potential for clinical translation. Herein, the >1000 nm tail emission of conventional biocompatible NIR cyanine dyes with emission peaks at 700-900 nm is systematically investigated, and a type of bright dye for NIR-II imaging with high potential for accelerating clinical translation is identified. The asymmetry of the π domain in the S state of NIR cyanine dyes is proven to result in a twisted intramolecular charge-transfer process and NIR-II emission, establishing a general rule to guide future NIR-I/II fluorophore synthesis. The screened NIR dyes are identified to possess a bright emission tail in the NIR-II region along with high quantum yield, high molar-extinction coefficient, rapid fecal excretion, and functional groups amenable for bioconjugation. As a result, NIR cyanine dyes can be used for NIR-II imaging to afford superior contrast and real-time imaging of several biological models, facilitating the translation of NIR-II bioimaging to clinical theranostic applications.

Although the kinetics of CF formation/ dissolution is still unclear, it is widely accepted that the CF formation/dissolution is strongly related to the electromigration and electrochemical reaction of anion (i.e., oxygen vacancy) [13][14][15][16] or cation (i.e., Cu 2+ , Ag + or Ni 2+ ). [17][18][19][20][21][22] Generally, RS behavior can be classifi ed as two modes: nonvolatile memory switching (MS) and volatile threshold switching (TS). In the MS mode, both LRS and HRS can be maintained after removing the external voltage, while the LRS in the TS mode will be back to the HRS once the applied voltage is smaller than a critical value. [23][24][25] To avoid confusion with MS, the LRS and HRS in TS are renamed as "TS ON-state" and "TS OFFstate" in this article. The MS device can be used for the non-volatile data storage [1][2][3][4][5] while TS device can be as a selector in series with memory cell to suppress crosstalk effect in the crossbar array. [26][27][28][29][30] Recently, some groups reported that TS and MS can coexist and mutually transform in a single device at suitable external excitation. [23][24][25][26][27][28] Several models have been proposed to explain this phenomenon, including CF thermal instability, [ 23 ] strong electron correlation effect, [ 24 ] quantum-wire model, [ 25 ] interface barrier modulation, [ 26 ] and space charge effect. [ 27 ] However, the underlying mechanism of the phenomenon is still unclear, especially lacking of direct evidences to uncover when and how the two RS modes happen and what is the internal relationship between them.Here, we demonstrate that the TS and MS modes can be modulated in the Ag/SiO 2 /Pt structure by controlling the compliance current ( I CC ) in electroforming. We systematically investigate the morphologies, chemical components, and dynamic growth of the CF using scanning electron microscope (SEM), high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) analysis. The results confi rm that the TS and MS modes correspond to the CF consisting of isolated and continuous Ag nanocrystals, respectively. In addition, by Kelvin probe force microscopy (KPFM) studies, the voltage potential distribution of CF in the ON-and OFF-state further indicate that the TS mode is Volatile threshold switching (TS) and non-volatile memory switching (MS) are two typical resistive switching (RS) phenomena in oxides, which could form the basis for memory, analog circuits, and neuromorphic applications. Interestingly, TS and MS can be coexistent and converted in a single device under the suitable external excitation. However, the origin of the transition from TS to MS is still unclear due to the lack of direct experimental evidence. Here, conversion between TS and MS induced by conductive fi lament (CF) morphology in Ag/SiO 2 /Pt device is directly observed using scanning electron microscopy and high-resolution transmission electron microscopy. The MS mechanism is related to the formation and dissolution of CF consisting of continuous Ag...